Electropolishing steel



Patented Aug. '21, 1951 ELECTROPOLISHING STEEL Christian J. Wernlund, Niagara Falls, N. Y., as-

signor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application March 15, 1949, i

1 Serial N 0. 8 1,630

9 Claims. (01. 2o4 14o.5)'

This invention relates to the electropolishing of steel by anodic action in a new and improved electrolyte.

In the metal finishing industry, it is generally desirable to give metal articles smooth polished surfaces. This is particularly true of electroplated surfaces having an exterior coating of nickel or chromium plate. Polishing by means of abrasives generally requires a great deal of skilled manual labor and is expensive. Given a smooth base metal surface, it is possible to electrodepcsit metal such as copper, nickel and the like, in a smooth, bright condition, rendering mechanical polishing of the electroplated surface unnecessary. However, in such cases the base metal must be polished in order that the electrodeposit have a smooth polished appearance. Various electrolytic polishing processes, generally consisting of anodic treatment in acidic solutions,

have been proposed for electropolishing steel and some of these have proved commercially practicable for electropolishing stainless steel and other types of alloy steels. However, most of the metal articles which require polishing prior to electroplating, e. g., automobile parts and the like, generally are made of unalloyed carbon steels. Heretofore, there has been available no commercially satisfactory process for electrolytic polishing of such carbon steels.

A difliculty which is inherent in many processes heretofore proposed for electropolishing steel is that the iron which is dissolved from the articles being electropolished is not electrolytically deposited on the cathode. This results in building up a high iron concentration, or precipitation of iron or iron compounds, in the electrolyte. This results in depletion of anions, requiring replenishment and excess formation of sludge which must be removed. There has been a need in industry for an efhcient electropolishing process operable for unalloyed carbon steels as well as alloy steels, in which any iron dissolved from the articles being olished will be electrolytically deposited upon the cathode so as to avoid contamination of the electrolyte with excess iron compounds.

An object of the present invention is a new and improved process for the electrolytic polishing of steel; A further object is an anodic electropolishing process for the anodic treatment of steel and other metals in which metal dissolved from the article being treated will be deposited upon the cathode, avoiding excessive sludge formation permitting the electrolyte to be utilized indefinitely with little replenishment. Still other objects will be apparent from the following description of my invention.

The above objects are attained in accordance with the present invention by anodically treating steel articles in an electrolyte composed of the an aliphatic amine, with a limited amount of water. The electrolyte is prepared in accordance with my invention by reacting an amine with hydrofluoric acid to produce the hydrogen fluoride salt of the amine. The presence of water in the electrolyte is essential; but the water content must be limited to not more than about 30% by weight. The water content may vary from a small trace, such as 0.01% to about 30% by weight. If the reaction product of the amine with hydrogen fluoride is a liquid, it is generally not necessary to add any water, as the small amount of water required to make the process operable will be absorbed by the reaction product from the air, if not already present in the amine or the hydrogen fluoride. In some cases, where the amine-hydrogen fluoride reaction product is a solid at operating temperatures it may be liquefied by adding up to about 30% by weight of water. The electropolishing bath may be operated at room temperature, but best results are generally obtained at elevated temperatures; and I refer to operate the bath at a temperature of -100" C. The nitrogen bases suitable for practicing my invention are selected from the heterocyclic nitrogen compounds having strongly basic properties, cyclohexylamine, and a variety of aliphatic and aromatic-diphatic amines, as illustrated by the examples hereinafter. Examples of suitable heterocyclic nitrogen bases are: pyridine, pyrrolidine, alpha-picoline, beta-picoline, piperazine and morpholine. The aliphatic amines may be primary, secondary or tertiary, and include alkylene polyamines and alkyl amines and alkylene polyamines in which the alkyl and alkylene groups may contain as substituent groups, phenyl, hydroxyl or amino groups. Generically, suitable aliphatic (including aromatic-aliphatic) amines may be represented by the type formula:

where a, b, c, d and e each represents hydrogen, alkyl hydroxyalkyl, aminoalkyl and phenyl.

In addition to the above described chemical character, suitable nitrogen bases must react with hydrofluoric acid to form hydrofluoride salts which are liquids in the presence of not more than about 30% by weight of water, at a temperature below the decomposition temperature. Preferably, the hydrofluoride should be liquid at a temperature not higher than about C.

From the standpoint of availability and cheapness, as well as electropolishing results, the preferred organic bases are morpholine, triethylamine, triethanolamine, diethanolamine and ethanolamine. These, and other suitable organic bases, may be used either singly or in any desired mixture thereof. An especially suitable organic base material is the common technical grade of triethanolamine, which. contains minor proportions of ethanolamine and diethanolamine.

The following examples illustrate specific modes of operating my invention.

EXAMPLE I 5'7 g. diethylethanolamine 23 g. hydrofluoric acid (100%) 20 g. water polished at a current density of 150 A/SF at 80 C. for minutes.

EXAMPLE II 56 g. triethylamine 22 g. hydrofluoric acid (100%) 22 g. water polished at a current density of 200 A/SF at 80 C. for 10 minutes.

EXAMPLE III 70 g. 2-amino-2-methyl-1,3-propanediol 26 g. hydrofluoric acid 30 g. water polished at a current density of 200 A/SF at 80 C. for 10 minutes.

EXAMPLE IV 68 g. 1-dimethylamino-2-propanol 16 g. hydrofluoric acid g. water polished at a current density of 125 A/SF at 80 C. for 10-15 minutes.

EXAMPLE V 80 g. N,N-dimethylani1ine 30 g. hydrofluoric acid 25 g. water polished at current density of 150 A/SF at 80 C. for 15 minutes.

EXAMPLE VI 43 g. piperazine 40 g. hydrofluoric acid 40 g. water polished at current density-of 125 A/ SF at 80 C. for 10 minutes.

EXAMPLE VII 75 g. morpholine 30 g. hydrofluoric acid 20 g. water polished at current density of 75 A/SF at 80 0. for 5 to 15 minutes.

The electrolytes utilized in the above examples were prepared by adding an aqueous solution of hydrogen fluoride to the amine with constant Crank-hole covers.

.4 electrolyte was electrolyzed between two steel electrodes, both being low carbon steel containing about 0.2% by weight of carbon and substantially no alloying elements such as chromium. titanium or the like. The anodes were strips of steel which had been polished with an approximately 180 grit polish. After the anodic treatment as indicated above, the anodes, after washing, were found to have a smooth, bright surface, substantially free from polishing marks.

EXAMPLE X An electrolyte made by reacting an aqueous hydrogen fluoride solution with triethanolamine in the following proportions:

Hydrogen fluoride 400 Water 450 Triethanolamine 1 1490 This was electrolyzed with steel cathodes at a temperature of C. and at a current density of 20 to 200 amps. per square foot to treat anodically the following steel articles:

Article Steel Composition Ford wrench about 0.1 to 0.2% carbon steel about 0.1% carbon steel 0.4% carbon steel.

0.9 to 1.35% carbon steel (High carbon). 0.15% carbon; 5% nickel.

0.35% carbon; 3%% nickel; l %chromiurn. 0.45% carbon; 0.8% chromium.

18% nickel; 8% chromium.

(Low carbon).

8. A. E. 1041 steel File S. A. E. 2515 steeL. S. A. E. 3335 steel...

8. A. E. 5145 steel. Stainless steel The above articles thus anodically treated all received good bright polished surfaces.

EXAMPLE XI Hydrogen fluoride reaction products of the amines tabulated below were made by reacting hydrofluoric acid with the amines in the presence of not more than 30% by weight of water. In most cases an aqueous solution of hydrofluoric acid was stirred into the amine. In some cases anhydrous hydrofluoric acid was mixed with the amine to produce the electrolyte. The resulting electrolytes were electrolyzed using steel anodes and steel cathodes. In each case the steel anode received a good polish. The amines thus tested were as follows:

I. Aliphatic amines A. PRIMARY AMINES Ethylamine Isopropyl amine n-Butylamine Sec-butylamine n-Amylamine Cyclohexylamine B. SECONDARY AMINES Diethylamine Diamylamine Dicyclohexylamine C. TERTIARY AMINES Trimethylamine Triethylamine 'Iriamylamine II. Aliphatic polyamines Ethylene diamine Diethylene triamine stirringuntil the reaction was completed. The (Q 1.3-dia1ninopropane at least per hour.

III. Functionally substituted aliphatic amines Ethanolamine Diethanolamine Triethanolamine Methyldiethanolamine Ethyldiethanolamine Dimethylethanolarnine Diethylethanolamine Isopropanolamine 1-dimethylamlno-2-propanol 2-amino-2-methyl-1,3-propanediol Z-aminoethyl ethanol amine Benzylamine IV. Aliphatic-aromatic amines N-methylaniline N,N-dimethylaniline N,N-diethylaniline N-butylaniline V. Functionally substituted aliphaticaromatic amines Phenyl ethanolamine Phenyl diethanolamine Other amines, namely, heptylamine, octylamine, decylamine and benzidine, when reacted with hydrogen fluoride formed solid reaction products which remained solid when heated to a temperature of about 100 C. in the presence of not more than 30% by weight of water, and hence were unsuitable for use as electrolytes at that temperature.

In the practice of my invention it is essential that the electrolyte contain not more than about 30% by weight of water and preferably not more than about 20%. In most cases it is preferable to have present about 2 to 20% byweight of Water. Smaller concentrations of water markedly decrease the conductivity of the electrolyte requiring a higher voltage for operation. If the water concentration is too high, the anodic steel article is not electropolished but is corroded and pitted giving a dull appearing surface. In most cases best results are obtained with a water content of about 5 to 20%.

I generally prefer to maintain the electrolyte, during the electropolishing operation, at a temperature of about 70 to 100 -C. but the invention is not restricted thereto. Lower temperatures reduce the .conductivity of the electrolyte and in some cases a minimum temperature is required to maintain the amine-HF compound in the liquid state. Temperatures as low as room temperature may be used in some cases, for example triethanolamine. The upper limit of the temperature for practical operation, is limited only by the nature of the amine utilized, as the temperature must be maintained below that at which the amine will become decomposed or volatilizes excessively. For example, low boiling amines such as triethylamine or morpholine require an operating temperature below about 60 C. On the other hand, triethanolamine, which is a higher boiling amine, may be operated at temperatures as high as 130 C. without excessive volatilization. By excessive volatilization I mean evaporation from an open vessel at the rate of I, therefore, prefer to operate at any temperature between the melting point of the amine-HF compound and the temperature at which excessive'volatilization or decomposition occurs, whichever is lower. If desired, however, the operation may be carried out above the temperature of excessive volatilization,

by enclosing the bath container, and condensing and returning volatile material by means 01' a reflux condenser. In such cases, oi course, the bath temperature must be below the decomposition temperature oi. the organic nitrogen hy drogen fluoride salt.

The relative proportions of hydrogen fluoride and the organic nitrogen base may be varied considerably. I prefer to use an excess of hydrogen fluoride and to avoid an excess of free base. Good results are obtained using, for example, from 1 to about 3 moles of hydrogen fluoride to 1 mole of a monoamine. When a polyamine is used, correspondingly higher amounts of hydrogen fluoride are preferred. That is, for each basic nitrogen atom in the base, there should be at least 1 mole (e. g. 1 to 3 moles) of hydrogen fluoride in the composition. Thus, when a diamine is used, there may be from 2 to 6 moles of hydrogen fluoride to 1 mole of the amine. there should be not less than 3 moles of hydrogen fluoride, or 3 to 9 moles of hydrogen fluoride, per mole of the amine. In other words, the molar ratio of total hydrogen fluoride (free and combined) to amine may be expressed by the formula: from n/l to 3n/1, where n equals the number of basic nitrogen atoms in the amine. The best results generally are obtained when the molar ratio is 1.5n/1 to 2.5n/1. By "basic nitrogen atom, I mean a nitrogen atom sufflciently basic to react with hydrogen fluoride to form a stable hydrogen fluoride salt. Such nitrogen atoms may be the nitrogen components of primary, secondary or tertiary amines.

The electrolyte may be prepared by reacting hydrogen fluoride with the amine, in the proportions indicated above, in the presence of 0.01 to 30% by weight of water. If desired, larger amounts of water may be present during the reaction, in which case I evaporate off sufflcient water to produce a liquid electrolyte containing 0.01 to 30% by weight of water.

During extended periods of operation, some loss of hydrogen fluoride may occur, which will cause a diminution in the conductivity of the electrolyte. When the conductivity decreases it may be readjusted by adding hydrofluoric acid or anhydrous hydrogen fluoride, as required. For example, I have maintained substantially constant conductivity by adding hydrogen fluoride in amount equivalent to about 1 lb. of 47% hydrofluoric acid per gallon of electrolyte for each 600 amperes/hour of electrolysis.

The herein described electropolishing process is useful for electropolishing malleable iron and any type of steel containing from 0.1 to 1.35% of carbon. It is likewise useful for electropolishing the so-called alloy steels which contain alloyed therewith elements such as chromium, nickel. titanium, etc. I have also found my process useful for polishing certain nonferrous metals, namely, copper, brass, electrolytically deposited chromium and aluminum. It does not operate satisfactorily to electropolish zinc, nickel, Monel metal or ordinary cast iron.

An important advantage of my process when utilized to electropolish steel is that most of the iron removed from the article being anodically treated is deposited upon the cathode and there is no substantial build-up of iron salts in the elec trolyte. Consequently the electrolyte may be utilized for an indefinite period of time and for its maintenance requires only replenishment to replace electrolyte removed by dragout or by volatilization and the addition of a relatively small Likewise, if a triamine is used,

amount of hydrofluoric acid. It has the further advantage that it is adapted for electropolishing all grades of iron and steel particles without changing the electrolyte composition or conditions of electrolysis. Thus iron or .steel articles, which vary in composition and character, may be treated successively or simultaneously in the same electrolyte. The process produces excellent polished steel surfaces which are well suited for electroplating and other purposes where a polished surface is required. The process also can be used to remove oxide scale from the metals it is adapted to polish.

I claim:

1. The process for electropolishing metal articles selected from the group consisting of malleable iron, steels, copper, brass, aluminum and electrolytically deposited chromium which comprises anodically treating said articles in an electrolyte consisting of 0.01 to 30% by weight of water and the hydrogen fluoride salt of an organic nitrogen base capable of reacting with hydrogen fluoride to form a salt which is liquid in the presence of 0.01 to about 30% by weight of water at a temperature below its decomposition temperature, said base being selected from the group consisting of cyclohexylamine, the heterocyclic organic bases and amines having the type formula:

where a, b, c, d and 6 each represent a radical selected from the group consisting of hydrogen, alkyl, hydroxyalkyl, aminoalkyl and phenyl.

2. The process for electropolishing metal articles selected from the group consisting of malleable iron, steels, copper, brass, aluminum and electrolytically deposited chromium which comprises anodically treating said articles in an electolyte consisting of 0.01 to 30% by weight of water and the hydrogen fluoride salt of an organic nitrogen base capable of reacting with hydrogen fluoride to form a salt which is a chemically stable liquid in the presence of 0.01 to about 30% by Weight of water at a temperature not higher than about 130 C., said base being selected from the group consisting of cyclohexylamine, the heterocyclic organic bases and amines having the type formula:

a d h-d-b/ where a, u, c, d and e each represent a radical selected from the group consisting of hydrogen, alkyl, hydroxyalkyl, aminoalkyl and phenyl.

3. The process for electropolishing steel articles which comprises anodically treating said articles in an electrolyte consisting of 0.01 to 30% by weight of water and the reaction product of hydrogen fluoride and an amine capable of reacting with hydrogen fluoride to form a salt which is a chemically stable liquid at ,a temperature not higher than about 130 C., and containing from n to 3n moles of hydrogen fluoride per mole of said amine where n represents the number of basic nitrogen atoms in said amine, said amine having the type formula:

d aaN I represents the number of basic nitrogen atoms in said amine.

5. The process for electropolishing steel articles which comprises anodically treating said articles in an electrolyte consisting of the reaction product of triethanolamine and hydrogen fluoride and containing from 1 to about 3 moles of hydrogen fluoride per mole of said amine and 0.01 to 30% by weight of water, at a temperature of about 70 to C.

6. The process for electropolishing steel articles which comprises anodically treating said articles in an electrolyte consisting of the reaction product of triethylamine and hydrogen fluoride and containing from 1 to about 3 moles of hydrogen fluoride per mole of said amine and 0.1 to 30% by weight of water, at a temperature of about 70 to 100 C.

'7. The process for electropolishing steel articles which comprises anodically treating said articles in an electrolyte consisting of 0.01 to 30% by weight of water and the reaction product of hydrogen fluoride and a heterocyclic nitrogen base capable of reacting with hydrogen fluoride to form a chemically stable salt liquid at a temperature not higher than C. and containing from n to about 312 moles of hydrogen fluoride per mole of said base, where n represents the number of basic nitrogen atoms in said base.

8. The process for electropolishing steel articles which comprises anodically treating said articles in an electrolyte consisting of 0.01 to 30% by weight of water and the reaction product of hydrogen fluoride and morpholine and containing from 1 to about 3 moles of hydrogen fluoride per mole of morpholine and 0.01 to about 30% by weight of water, at a temperature of about 70 to 100 C.

9. The process for electropolishing steel article which comprises anodically treating said articles in an electrolyte consisting of the reaction product of an alkylamine and hydrogen fluoride containing from 1 to about 3 moles of hydrogen fluoride per mole of said amine and 0.01 to 30% by weight of water, at a temperature of about 70 to 100 C.

CHRISTIAN J. WERNLUND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,040,618 Mason et al May 12, 1936 2,411,410 Beckwith Nov. 19, 1946 OTHER REFERENCES Metal Finishing, (New York), June 1942, pages 306 thru 312.

Certificate of Correction Patent No. 2,565,189 August 21, 1951 CHRISTIAN J. WERNLUND It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 38, for excess read excessive; column 7 line 43, for electolyte read electrolyte; column 8, line 1, before represent insert each; line 28, for 0.1 read 0.01;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Ofiioe.

Signed and sealed this 16th day of October, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Paton. 

1. THE PROCESS FOR ELECTROPOLISHING METAL ARTICLES SELECTED FROM THE GROUP CONSISTING OF MALLEABLE IRON, STEELS, COPPER, BRASS, ALUMINUM AND ELECTROLYTICALLY DEPOSITED CHROMIUM WHICH COMPRISES ANODICALLY TREATING SAID ARTICLES IN AN ELECTROLYTE CONSISTING OF 0.01 TO 30% BY WEIGHT OF WATER AND THE HYDROGEN FLUORIDE SALT OF AN ORGANIC NITROGEN BASE CAPABLE OF REACTING WITH HYDROGEN FLUORIDE TO FORM A SALT WHICH IS LIQUID IN THE PRESENCE OF 0.01 TO ABOUT 30% BY WEIGHT OF WATER AT A TEMPERATURE BELOW ITS DECOMPOSITION TEMPERATURE, SAID BASE BEING SELECTED FROM THE GROUP CONSISTING OF CYCLOHEXYLAMINE, THE HETEROCYCLIC ORGANIC BASES AND AMINES HAVING THE TYPE FORMULA: 